Helium production in lng plants

ABSTRACT

A process for separating a helium-rich fraction from a liquefied natural gas stream, is disclosed. In an embodiment, the process includes expansion of a liquefied natural gas stream and separation of the helium-rich fraction. The helium-rich fraction is heated countercurrent to a natural gas stream to be cooled and liquefied. The natural gas stream liquefied in a heat exchange countercurrent to the helium-rich fraction to be heated is fed prior to and/or in the separation of the helium-rich fraction. The total enthalpy of the mixture of the two aforementioned natural gas streams brought to the separation of the helium-rich fraction is variable.

BACKGROUND AND SUMMARY OF THE INVENTION

This application claims the priority of International Application No. PCT/EP2006/001805, filed Feb. 28, 2006, and German Patent Document No. 10 2005 010 053.8, filed Mar. 4, 2005, the disclosures of which are expressly incorporated by reference herein.

The invention relates to a process for separating a helium-rich fraction from a liquefied natural gas stream.

Helium is normally extracted in great quantities from natural gas or from natural gas fractions—such as accumulate, for example, in what are known as LNG baseload plants, from a gas mixture consisting essentially of methane, a high percentage of nitrogen and hydrocarbons.

Smaller amounts of helium can also be separated and thus extracted from the air in cryogenic air fractionating plants using what is known as low-temperature air fractionation. Helium occurs in known natural gas deposits in the amount of up to about 0.2% mole. For this reason, technical extraction makes sense only as part of the aforementioned LNG baseload plants since the inert helium is concentrated in them in the flash gas of the LNG storage tanks. When extracting helium at what is known as the “cold end” of LNG baseload plants, it is desirable to extract a constant volume of helium even with different compositions of natural gas although primarily the different concentrations of nitrogen of the natural gas result in different flash conditions for the helium-rich flash. Normally, the liquefied natural gas present at high pressure, whose temperature is almost determined because of the refrigerant(s) from the LNG baseload plant, is initially reduced to a median pressure between 3 and 10 bar, the helium-rich flash gas obtained thereby—which typically has a helium content of between 5% and 20%—is heated and taken to a helium extraction plant as a feedstock fraction.

Prior to its being returned, the helium-rich flash gas is heated—for example, countercurrent to a purified, natural gas stream present under high pressure—which is drawn off before the actual liquefaction part of the liquefaction process and thus originates from what is known as the “warm area” of the liquefaction process—in order to be able to utilize the coldness of the helium-rich flash gas to cool and liquefy this additional natural gas flow. The volume of this additional natural gas flow must be selected such that there is no noticeable change in the liquefaction performance of the LNG baseload plant, which is the case in a broad range of volume flow.

Using this process, however, different qualities of natural gas cannot be considered with respect to the content of helium and nitrogen which in turn result in great differences with respect to the helium and the nitrogen content in the flash gas separated from the liquefied natural gas. The object of the present invention is to specify a generic process for separating a helium-rich fraction from a liquefied natural gas stream which avoids the aforementioned disadvantages.

To achieve this objective, a generic process is provided which comprises the following process steps:

a) expanding the liquefied natural gas stream and separating a helium-rich fraction,

b) heating the helium-rich fraction countercurrent to a natural gas stream to be cooled and liquefied, and

c) feeding of the natural gas stream liquefied in the heat exchange countercurrent to the helium-rich fraction to be heated prior to and/or in the separation of the helium-rich fraction,

d) where the total enthalpy of the mixture of the two aforementioned natural gas streams brought to the separation of the helium-rich fraction can be varied.

The volume stream of the aforementioned natural gas stream to be cooled and liquefied is preferably adjusted such that no essential change results in the liquefaction performance of the LNG baseload plant.

In principle, the total enthalpy of the mixture of the two aforementioned natural gas streams taken to the separation of the helium-rich fraction—this a two-phase stream—can happen by:

varying the supercooling conditions of the liquefied natural gas at what is known as the “cold end” of the liquefaction process; however, this would require intervening in the operation of the LNG baseload plant, which is normally not desirable,

heating the supercooled natural gas stream from the “cold end” of the LNG baseload plant countercurrent to one or more refrigerant streams; even this version would result in a normally undesirable intervention in the operation of the LNG baseload plant, or

heating the supercooled natural gas stream from the “cold end” of the LNG baseload plant by admixing a hotter natural gas stream from the “hot end” of the LNG baseload plant; this alternative results in an increase in the throughput through the LNG baseload plant, for which reason this alternative is preferred.

The process in accordance with the invention now makes it possible to cope with a wide variety in the helium and nitrogen contents in the natural gas stream to be liquefied and the liquefied natural gas stream. The helium-rich fraction and the natural gas stream to be cooled and liquefied which are brought together in the exchange of heat can now be heated or cooled, selectively temperature-controlled with respect to each other. Thus the conditions for the expansion of the liquefied natural gas stream and the separation of the helium-rich fraction can be selectively regulated so that a maximum separation or yield of helium is possible for different compositions of liquefied natural gas streams through the expansion and separation of the helium-rich fraction.

Further developing the process in accordance with the invention, it is provided that—consistent with the composition of the natural gas streams—the volume flow of the helium-rich fraction taken to the heat exchange and/or the volume flow of the natural gas stream taken to the heat exchange to be cooled and liquefied is varied in such a way that the helium yield from the helium-rich fraction remains essentially constant and/or is maximized.

Additional advantageous embodiments of the process in accordance with the invention are characterized in that:

the helium-depleted, liquefied natural gas stream is expanded and undergoes fuel-gas separation,

the fuel-gas fraction extracted in the fuel-gas separation is heated countercurrent to the natural gas stream to be cooled and liquefied,

at least one partial stream of the natural gas stream to be cooled and liquefied, at least one partial stream of the helium-rich fraction to be heated and/or at least one partial stream of the fuel-gas fraction to be heated is taken past the heat exchange between the helium-rich fraction to be heated and the natural gas stream to be cooled and liquefied,

the heat exchange between the helium-rich fraction to be heated and the natural gas stream to be cooled and liquefied takes place in at least one coil heat exchanger and/or at least one TEMA heat exchanger,

the separation of the helium-rich fraction takes place in a separator or a wash column.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a first embodiment of the invention.

FIG. 2 illustrates a second embodiment of the invention.

DETAILED DESCRIPTION OF THE DRAWINGS

Via line 1—as shown in FIG. 1—a liquefied natural gas stream which was extracted in any kind of natural gas liquefaction process is brought in, expanded in valve a to a pressure between 3 and 10 bar and then taken to separator D via line 2. A helium-rich gas fraction is withdrawn from the head of this separator D via line 3.

The helium-rich gas fraction is heated in heat exchanger E, which is preferably a coil heat exchanger or a TEMA heat exchanger, countercurrent to a natural gas stream to be cooled and liquefied, which will be discussed in greater detail in what follows, and then via line 4 taken for further use, such as a process in which a pure helium fraction is extracted.

From the sump of the separator D, a helium-depleted liquid fraction is drawn off via line 5, expanded in valve b to a pressure between 1 and 5 bar and taken via line 6 to its further application—if necessary, following previous transfer by means of a pump and intermediate storage in a storage tank at atmospheric pressure.

The natural gas stream mentioned which is to be cooled and liquefied is taken via line 9 to heat exchanger E. This gaseous natural gas stream is, for example, drawn off from the natural gas liquefaction process following the usually necessary separation of heavy hydrocarbons. The volume of this natural gas stream is preferably adjusted such that no noticeable change in the liquefaction performance of the LNG baseload plant results as a consequence of the helium separation D.

By exploiting the cold from the helium-rich fraction supplied by line 3 to the heat exchanger E, the natural gas stream fed to the heat exchanger E via line 9 is now cooled and liquefied. It is then admixed via line 10, in which an expansion valve c is provided, to the liquefied natural gas stream in line 2 before being taken to separator D.

Depending on the mixture temperature and pressure, different helium concentrations and volumes result in the helium-rich gas fraction 3 drawn off at the head of the separator D.

In FIG. 1, two additional by-pass lines are shown in each of which a control valve d and e are located. Through these by-pass lines 7 and 11 the fractions taken in lines 3 and 9 can be passed completely or at least partially to heat exchanger E.

In accordance with the invention, the volume flow of the natural gas stream taken to heat exchanger E via line 9 by means of the expansion valve c and/or the by-pass line 11 can be varied. The same applies to the helium-rich fraction brought to heat exchanger E via line 3 since its volume flow can be regulated by heat exchanger E by means of the by-pass line 7.

By means of the aforementioned control mechanisms, maximum helium yields or volumes can be adjusted or obtained even for different compositions of the liquefied natural gas stream in the helium-rich fraction 3 drawn off at the head of the separator D.

In accordance with the volume, the composition, the degree of supercooling and the pre-pressure and thus the total enthalpy of the liquefied natural gas stream brought in over line sections 1 and 2 and the volume, the composition, the pressure and the temperature and thus the total enthalpy of the natural gas stream brought in over lines 9 and 10, the supercooling temperature of the last-named stream after the exchange of heat E is adjusted in order to maximize the helium yield in the helium-rich fraction 3.

The purpose of this procedure is to adjust the conditions in the separator D, meaning the entire enthalpy of the mixture, in such a way that even with different compositions of the natural gas streams 1 and 9 a maximum helium yield in the helium-rich flash gas 3 and 4 is achieved and at the same time the production of the LNG baseload process is not affected or affected only minimally. In this way an optimal starting fraction can be prepared for a downstream process for a pure helium extraction.

If comparatively low temperature differences between 5 and 30 K occur in heat exchanger E, the heat exchanger is preferably designed as a plate exchanger. In the event of greater temperature differences it is advantageous to implement heat exchanger E as a coil heat exchanger and/or TEMA heat exchanger.

In particular when the volumes of the streams brought in over line sections 1 and 2 or 9 and 10 remain approximately constant over time, but their helium and nitrogen content varies, the supercooling temperature of the natural gas stream brought in via line sections 9 and 10 necessary for a maximum helium yield can be adjusted and controlled by means of by-pass line 11 at the outlet of heat exchanger E.

Using the embodiment shown in FIG. 1 of the process in accordance with the invention, however, a maximum of 97% of the helium contained in the natural gas stream can be extracted.

If—as is shown in FIG. 2—the separator D is replaced by a wash column (K), a helium yield of up to 99.9% can be realized.

To do this, it is necessary to take the natural gas stream liquefied in heat exchanger E′—which is cooled countercurrent to the helium-rich gas fraction 3′ which is to be heated—via line 10′ to the wash column (K) as a strip stream while the liquefied natural gas stream expanded in valve a′, is given up via line 2′ to the wash column (K) as reflux.

This increase in the helium yield certainly requires an increase in the costs for equipment and technology but appears acceptable in view of the value of helium.

The process in accordance with the invention for separating a helium-rich fraction from a liquefied natural gas stream makes it possible to maximize the helium yields from the most highly varied liquefied natural gas streams. The required investment for controls can be kept within bounds so that implementing the process in accordance with the invention results in only insignificant additional costs. 

1-6. (canceled)
 7. A process for separating a helium-rich fraction from a liquefied natural gas stream, comprising the steps of: a) expansion of a liquefied natural gas stream and separation of the helium-rich fraction; b) heating of the helium-rich fraction countercurrent to a natural gas stream to be cooled and liquefied; and c) feeding of the natural gas stream liquefied in a heat exchange countercurrent to the helium-rich fraction to be heated prior to and/or in the separation of the helium-rich fraction; d) wherein the total enthalpy of the mixture of the two aforementioned natural gas streams brought to the separation of the helium-rich fraction is variable.
 8. The process according to claim 7, wherein a temperature of the natural gas stream liquefied in the heat exchange countercurrent to the helium-rich fraction to be heated is variable.
 9. The process according to claim 7, wherein a volume flow of the helium-rich fraction taken to the heat exchange and/or a volume flow of the natural gas stream taken to the heat exchange to be cooled and liquefied, is varied such that a helium yield from the helium-rich fraction remains essentially constant and/or is maximized.
 10. The process according to claim 7, wherein at least one partial stream of the natural gas stream to be cooled and liquefied and/or at least one partial stream of the helium-rich fraction to be heated is taken past the heat exchange between the helium-fraction to be heated and the natural gas stream to be cooled and liquefied.
 11. The process according to claim 7, wherein the heat exchange between the helium-rich fraction to be heated and the natural gas stream to be cooled and liquefied takes place in at least one coil heat exchanger and/or at least one TEMA heat exchanger.
 12. The process according to claim 7, wherein the separation of the helium-rich fraction is implemented in a separator or a wash column.
 13. A process for separating a helium-rich fraction from a liquefied natural gas stream, comprising the steps of: expanding a liquefied natural gas stream; cooling and liquefying a natural gas stream in a heat exchanger; providing the liquefied natural gas stream and the cooled and liquefied natural gas stream as a mixture to a separator; and separating a helium-rich fraction from the mixture in the separator; wherein the helium-rich fraction is provided from the separator to the heat exchanger for cooling and liquefying the natural gas stream and wherein a total enthalpy of the mixture is variable.
 14. The process according to claim 13, further comprising the steps of controlling a volume of the natural gas stream cooled and liquefied in the heat exchanger and controlling a volume of the helium-rich fraction provided from the separator to the heat exchanger.
 15. The process according to claim 13, further comprising the step of controlling a yield of helium in the helium-rich fraction by adjusting a temperature of the cooled and liquefied natural gas stream.
 16. The process according to claim 14, wherein the step of controlling the volume of the natural gas stream cooled and liquefied in the heat exchanger and the step of controlling the volume of the helium-rich fraction provided from the separator to the heat exchanger include the step of bypassing a portion of the natural gas stream and the helium-rich fraction around the heat exchanger. 